1. Field of the Invention
The present invention relates generally to a method of allocating communication resources in a broadband wireless communication system, and in particular, to a method and apparatus for allocating subcarriers in a broadband wireless communication system using multiple carriers.
2. Description of the Related Art
The first MCM (Multi-Carrier Modulation) systems appeared in the late 1950's for military high frequency radio communication. OFDM (Orthogonal Frequency Division Multiplexing), a special case of MCM with overlapping orthogonal subcarriers, was initially developed in the 1970's. With OFDM, a serial symbol sequence is converted to parallel symbol sequences and modulated to mutually orthogonal subcarriers before transmission. Because of difficulty in orthogonal modulation between multiple carriers, OFDM has limited practical application to communication systems.
However, in 1971, Weinstein, et. al. proposed an OFDM scheme that applies DFT (Discrete Fourier Transform) to parallel data transmission as an efficient modulation/demodulation process, thereby creating a driving force for OFDM development. In addition, the introduction of a guard interval further reduced adverse effects of multi-path propagation and delay spread in communication systems. Although complex hardware created an obstacle to the widespread use of OFDM, recent advances in digital signal processing technology including FFT (Fast Fourier Transform) and IFFT (Inverse Fast Fourier Transform) have enabled practical OFDM implementation.
OFDM can be implemented for digital data communications such as DAB (Digital Audio Broadcasting), digital TV broadcasting, WLAN (Wireless Local Area Network), and W-ATM (Wireless Asynchronous Transfer Mode). OFDM also exhibits high frequency use efficiency, reduces effects of ISI (Inter-Symbol Interference) by guard intervals, and is robust against multi-path fading. Therefore, OFDM provides optimum transmission efficiency for high-speed data transmission.
OFDM-based multiple access techniques are divided into OFDMA (Orthogonal Frequency Division Multiple Access) and FH (Frequency Hopping)-OFDM. FH-OFDM is a combination of FH and OFDM. Both OFDMA and FH-OFDM commonly seek to achieve frequency diversity gain by distributing data tones across the total frequency band. OFDMA is a scheme of transmitting each OFDM symbol across a plurality of subcarriers that form one subchannel.
OFDMA applications for broadband wireless communication systems include systems defined in the IEEE 802.16a and IEEE 802.16e standards. Such OFDMA systems adopt a 2048-point FFT, for example that divides 1702 tones into 166 pilot tones and 1536 data tones. The 1536 data tones are further divided into 32 subchannels for allocation to users, each subchannel including 48 data tones. OFDMA is a multiple access technique where the frequency domain is divided into subchannels, each having a plurality of subcarriers. The time domain is divided into a plurality of time slots, and the subchannels are allocated to users.
The IEEE 802.16a or IEEE 802.16e system divides a broad frequency band of 10-MHz into subchannels on the frequency domain only. About 1600 to 1700 subcarriers are allocated to one OFDM symbol using 2048 points FFT. Thus, the number of cells can be increased depending on subcarrier allocation. Yet, only about 40 cells are identifiable when subchannels are formed by subcarrier allocation in the prior art, considering collision between channels in a multi-cell environment. To facilitate deployment of a broadband wireless network, the number of identifiable cells should be about 100. In this context, a conventional subchannel/subcarrier allocation method in an OFDMA scheme is limited in the number of cells it can service.